They define "weather whiplash" or "precipitation whiplash" as a transition from a much drier than normal year to a much wetter than normal year, such as the change that occurred between the winters of 2015-2016 and 2016-2017.
As one might expect, with such an explosive claim and graphic metaphor, the media would go wild over it. They did. Headlines describing the civilization-testing whiplash were found in media outlets across the world, from the front page of the LA Times to CNN and even made Dan Rather's blog. Environmental activists website covered this revelation in depth.
But I suspect most reporters did not read the paper, and as I will describe below, this work suffers from substantial problems and claims that are at the best excessive, even using the model simulations they describe. And the results are entirely based on model results that don't seem to match well with what has happened in the real world.
Swain et al.'s claims of a precipitation whiplash is based only on climate model output and so is only as good as the model. Specifically, they used a 40-member collection of forecasts (an ensemble) using the Community Earth System Model (CESM). This is a global model developed at the National Center for Atmospheric Research. I am quite familiar with CESM and have used its output in some of my own work. It is run at approximately 1° horizontal resolution (about 111 km) and thus is unable to simulate the details of western U.S. precipitation and weather systems.
Specifically, they only considered a whiplash from dry to wet conditions. For some reason they did not think a wet to dry whiplash was important.
Swain and colleagues looked at the distribution of precipitation in the model during a pre-industrial period (before 1850) and found the top 20% and bottom 20% precipitation years. With those numbers they examined CESM climate simulations, encompassing most of the 20th and 21st centuries, that was driven by an aggressive (probably too aggressive) increase of greenhouse gases (RCP 8.5 in climate modeler lingo), looking for the frequency of changes from dry (the bottom 20% before 1850) to wet (top 20% before 1850) years. And they did this for each ensemble member and thus were able to get the change in frequencies of this one-sided whiplash as the model simulations forced by increasing greenhouse gases.
Their results are found in the figure below for some model grid boxes in southern and northern CA (they don't really specify in the paper exactly where), showing the change in frequency of dry to wet precipitation whiplash events. A measure of the range of climate model whiplashes are shown by the purple shading (67% of the simulations are within the shaded areas and purple line shows the ensemble mean (which has been smoothed or averaged over time)).
So, what does their analysis of the models show?
For northern California, not much. A decline in whiplash in the 1970s, an increase from roughly 2015 to 2025, and not much change for the next 30 years. According to their own analysis, the changes in northern CA are NOT statistically significant. And keep in mind that most of California's reservoir capacity is in the north.
For southern CA, there is an abrupt change from reducing whiplash to a very slow steady rise starting around 1985, with the whiplash remaining quite small until mid-century. Considering that the envelope of uncertainty encompasses zero change until around 2050, I am sure the change would NOT be statistically significant through that time. As shown later, this evolution in southern CA does NOT compare well with observations through 2018, where there is no trend. And in any case, southern CA has few large reservoirs and only a small amount of California agriculture.
In general, there paper shows not much change in the number of dry years but increasing wet years for the northern two thirds of the state as we proceed into the second half of the century.
What has Actually Occurred with Precipitation Whiplashes
Here is the average wet-season (November-March) precipitation for California from 1936-2018 from the NOAA/NWS Division data available from the wonderful NOAA ESRL website. This plot and subsequent plots were made by UW Atmospheric Sciences staff member, Neal Johnson. Why did we start in 1936? Because the ESRL website said there were issues before 1935.
Not much trend in CA precipitation, but plenty of ups and downs---now known as whiplash, I guess.
Anyway, since this is science, let's create a WHIPLASH INDEX (WI) that is simply difference between one year's precipitation and that of the year before (shown below). (Note, the correct pronunciation of WI is "why"). Now this is a bit different than Swain et al. whiplash plot (they showed the frequency of increases of precipitation between very dry years (bottom 20%) and wet years (top 20%). But my index is much easier to understand and is more informative. And my index is based on OBSERVATIONS in the REAL WORLD unlike the model-based whiplash guidance in the Swain et al. paper.
What is clearly apparent is that it is hard to find evidence of an increase in amplitude in the whiplash index based on observations. Now, if we look at the trend of increases in precipitation (above the dashed line)--which is very similar to Swain et al's approach-- there is no evidence of increasing dry to wet years. In fact, there are LESS of them during the past two decades...and more in the decades before.
Or look at the wet to dry transitions---no evidence of any trend.
But let's take this one step further, Swain et al. divided their whiplashes into southern and northern California. Let's do better than that (a UW Husky will always try to improve upon a UCLA Bruin)--- let's calculate the whiplash index for southern, central, and northern California (below).
These results are very important--they show there is NO increase in precipitation whiplash for any sub-region of California WHEN ONE LOOKS AT WHAT ACTUALLY HAPPENED.
In contrast, the model output applied in Swain et al shows that there is an increase in whiplash events starting around 1985 in southern CA and 2005 in northern CA. The clear conclusion is that something is amiss with the model in predicting California precipitation. And therefore, the predictions used in this paper are suspect.
The fact that the model is not getting the year by year variability correct for California precipitation is hardly surprising. West Coast precipitation is modulated by El Nino/La Nina and teleconnections (remote physical connections) forced by convection in the tropics. And such low-resolution models do a poor job on tropical convection and thus will mess up the teleconnections. I know a lot about this because I am actively working in this area. And there is a way to fix the problem...but it takes huge computer resources.
But even if their model output was correct, I would argue that they and certainly the media are way overhyping these results. In fact, at face value their predictions are good news for California. According to their results, the increase of whiplash events in northern California is slight and is not significantly significant at even the low 90% significant level (more studies use 95% significance). And most of the reservoir capacity in California is in the northern portion of the state.
They also show that there will be an increase in winter precipitation over time across most of California. Now California (unlike Washington) has huge multi-year storage capacity, so even if their model is right and there was more year to year variability, California would be fine....they simply will store the water from the wet years.
There are a lot of other technical issues with the paper I could bring up, but the bottom line is clear:
(1) observations over the past 80 years do not show an increase in precipitation whiplash in California even though the effects of global warming have begun.
(2) their model results are inconsistent with observed trends
(3) even if they are right, the implications are positive, not negative. California will have more water over time.
Now it is true that temperatures will increase and that will cause more evaporation and drying. Sierra snowpack will decline (but the water will still be there as rain). So California will probably want to add more reservoir capacity and waste less water (more drip irrigation, less crazy water loving crops). They could even push water underground during wet years. This is a solvable problem and not a existential threat. Weather whiplash will probably be the least of their problems.
As a result, the public is exposed to essentially incorrect information and gets turned off by another apocalyptic prediction. And such poor communication gets in the way of properly dealing with climate change, a serious issue, in a rational, fact-based way.